This invention relates to electric motor driven fluid handling assemblies and, more particularly, to a seal-less pump and motor assembly having improved electrical characteristics.
There are various applications in which a mechanical apparatus may be exposed or immersed in a fluid and adapted for being driven by an electric motor. Typical examples are a water pump in a dishwasher or clothes washing machine and an agitator in a clothes washing machine. In such applications, it is desirable to isolate the electric motor from the water both to protect the motor and to prevent electric shock hazards. A classic method of isolating the electric motor is to extend a shaft from the mechanical apparatus through a seal to the motor. The shaft to seal interface must provide for relative shaft motion and therefore is subject to wear and deterioration leading to fluid leaks at the interface.
An alternative strategy which avoids the potential seal leakage is to place the motor into the fluid environment. However, this strategy is inadvisable for water pumps and can be expensive when the electrical connections of the motor must be fluid proof.
Another method which avoids the seal leakage problem is to construct the apparatus, e.g., a pump, within a housing which also encompasses the motor rotor. The housing closely envelopes the circumference of the rotor without contact. The motor stator is then positioned outside the housing about the rotor. With a typical plastic housing, this arrangement requires a relatively large space between the rotor and stator, i.e., the effective "air gap" may be as much as 10 times the normal motor gap for an induction motor. For example, a minimum thickness for a plastic housing is generally about 0.09 inches while a nominal air gap for an efficient induction motor is about 0.01 inch. The resulting construction produces a motor which is oversized, expensive and inefficient with poor operating characteristics.
Still another prior art attempt to resolve the electric motor/pump problem of isolating the motor from the pumped fluid is to use a permanent magnet motor. Such a motor is expensive due to both the magnet cost and fabrication costs to meet water resistant constraints. Further, simple single phase permanent magnet synchronous motors are sometimes used for this purpose and are difficult to start in a controlled direction and have synchronization problems. If an electronically commutated control is used, the motor and drive cost increases dramatically.
Another challenge when designing a seal-less pump is that, even in relatively clean water, the wet rotor of a seal-less pump is subject to corrosion because of the presence of dissolved oxygen. A conventional technique for resisting corrosion is to coat the rotor with a material such as a plastic or an epoxy or to plate the rotor with a corrosion resistant metal such as aluminum. Crevices between rotor laminations and/or between rotor laminations and the rotor cage cause effective sealing to be difficult, and the coatings sometimes fail after a number of immersions.
To avoid the crevices, a solid iron rotor can be used. Sheet rotors comprising a copper shell brazed to a solid steel core are used in X-ray tube target rotators to withstand high temperatures, high speed, and vacuum conditions. Such rotors are typically coated with infra-red emitters.
Solid iron and steel cores can become corroded, and skin effects can affect electromagnetic steady state performance in the solid cores even at low slip frequencies. These skin effects can lead to difficulties in starting the rotor.